Spinal implant for insertion between vertebral bodies

ABSTRACT

An artificial spinal implant is disclosed for insertion between two adjacent vertebrae. Instrumentation and procedure is also disclosed.

RELATED APPLICATIONS

[0001] This application is a continuation of U.S. application Ser. No.10/685,776, filed Oct. 15, 2003; which is a continuation of U.S.application Ser. No. 08/480,684, filed Jul. 7, 1995; which is adivisional of application Ser. No. 07/968,240, filed Oct. 29, 1992, nowU.S. Pat. No. 5,741,253; which is a continuation of U.S. applicationSer. No. 07/698,674, filed May 10, 1991, now abandoned; which is adivisional application of U.S. application Ser. No. 07/205,935, filedJun. 13, 1988, now U.S. Pat. No. 5,015,247; all of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to an artificial fusion implant tobe placed into the intervertebral space left after the removal of adamaged spinal disc.

[0003] The purpose of the present invention is to provide an implant tobe placed within the intervertebral disc space and provide for thepermanent elimination of all motion at that location. To do so, thedevice is space occupying within the disc space, rigid, self-stabilizingto resist dislodgement, stabilizing to the adjacent spinal vertebrae toeliminate local motion, and able to intrinsically participate in avertebral to vertebra bony fusion so as to assure the permanency of theresult.

[0004] At present, following the removal of a damaged disc, either boneor nothing is placed into the space left. Placing nothing in the spaceallows the space to collapse which may result in damage to the nerves;or the space may fill with scar tissue and eventually lead to areherniation. The use of bone is less than optimal in that the boneobtained from the patient requires additional surgery and is of limitedavailability in its most useful form, and if obtained elsewhere, lacksliving bone cells, carries a significant risk of infection, and is alsolimited in supply as it is usually obtained from accident victims.Furthermore, regardless of the source of the bone, it is only marginalstructurally and lacks a means to either stabilize itself againstdislodgement, or to stabilize the adjacent vertebrae.

[0005] A review of related prior art will demonstrate the novelty ofthis present invention.

[0006] There have been an extensive number of attempts to develop anacceptable disc prosthesis (an artificial disc). Such devices by designwould be used to replace a damaged disc and seek to restore the heightof the interspace and to restore the normal motion of that spinal joint.No such device has been found that is medically acceptable. This groupof prosthetic or artificial disc replacements, seeking to preservespinal motion and so are different from the present invention, wouldinclude:

[0007] U.S. Pat. No. 3,867,728 STUBSTAD—describing a flexible discimplant.

[0008] U.S. Pat. No. 4,349,921 KUNTZ—describing a flexible discreplacement with file like surface projections to discourage devicedislocation.

[0009] U.S. Pat. No. 4,309,777 PATIL—describing a motion preservingimplant with spiked outer surfaces to resist dislocation and containinga series of springs to urge the vertebrae away from each other.

[0010] U.S. Pat. No. 3,875,595 FRONING—describing a motion preservingbladder-like disc replacement with two opposed stud-like projections toresist dislocation.

[0011] U.S. Pat. No. 2,372,622 FASSIO (French)—describing a motionpreserving implant comprising complementarily opposed convex and concavesurfaces.

[0012] In summary then, these devices resemble the present inventiononly in that they are placed within the intervertebral space followingthe removal of a damaged disc. In that they seek to preserve spinalmotion, they are diametrically different from the present inventionwhich seeks to permanently eliminate all motion at that spinal segment.

[0013] A second related area of prior art includes those devicesutilized to replace essentially wholly removed vertebra. Such removal isgenerally necessitated by extensive vertebral fractures, or tumors, andis not associated with the treatment of disc disease. While the presentinvention is to be placed within the disc space, these other vertebraldevices cannot be placed within the disc space as at least one vertebraehas already been removed such that there no longer remains a “discspace.” Furthermore, these devices are limited in that they seek toperform as temporary structural members mechanically replacing theremoved vertebra (not a removed disc), and do not intrinsicallyparticipate in supplying osteogenic material to achieve cross vertebraebony fusion. Therefore, again unlike the present invention whichprovides for a source of osteogenesis, use of this group of devices mustbe accompanied by a further surgery consisting of a bone fusionprocedure utilizing conventional technique. This group consisting ofvertebral struts rather than disc replacements would include thefollowing:

[0014] U.S. Pat. No. 4,553,273 WU—describing a turnbuckle-like vertebralstrut.

[0015] U.S. Pat. No. 4,401,112 REZAIAN—describing a turnbuckle-likevertebral strut with the addition of a long stabilizing staple thatspans the missing vertebral body.

[0016] U.S. Pat. No. 4,554,914 KAPP—describing a large distractiblespike that elongates with a screw mechanism to span the gap left by theremoval of an entire vertebra and to serve as an anchor for acryliccement which is then used to replace the missing bone (vertebra).

[0017] U.S. Pat. No. 4,636,217 OGILVIE—describing a vertebral strutmechanism that can be implanted after at least one vertebra has beenremoved and which device consists of a mechanism for causing theengagement of screws into the vertebra above and the vertebra below theone removed.

[0018] In summary then, this group of devices differs from the presentinvention in that they are vertebral replacements struts, do notintrinsically participate in the bony fusion, can only be inserted inthe limited circumstances where an entire vertebra has been removed fromthe anterior approach, and are not designed for, or intended to be usedfor the treatment of disc disease.

[0019] A third area of prior art related to the present inventionincludes all devices designed to be applied to one of the surfaces ofthe spine. Such devices include all types of plates, struts, and rodswhich are attached by hooks, wires, and screws. These devices differsignificantly from the present invention in that they are not insertedwithin the disc space, and furthermore do not intrinsically participatein supplying osteogenic material for the fusion.

[0020] Therefore, with these devices where permanent spinalimmobilization is desired an additional surgery consisting of a spinalfusion performed by conventional means for the use of supplementalmethylmethacrylate cement is required. Such devices applied to thespine, but not within the disc space, would include the following:

[0021] U.S. Pat. No. 4,604,995 STEPHENS—describing a “U” shaped metalrod attached to the posterior elements of the spine with wires tostabilize the spine over a large number of segments.

[0022] U.S. Pat. No. 2,677,369 KNOWLES—describing a metal column deviceto be placed posteriorly along the lumbar spine to be held in positionby its shape alone and to block pressure across the posterior portionsof the spinal column by locking the spine in full flexion therebyshifting the maximum weight back onto the patient's own disc.

[0023] Other devices are simply variations on the use of rods (e.g.,Harrington Luque, Cotrel-Dubosset, Zielke), wires or cables (Dwyer),plates and screws (Steffee), or struts (Dunn, Knowles).

[0024] In summary, none of these devices are designed or can be usedwithin the disc space, do not replace a damaged disc, and do notintrinsically participate in the generation of a bony fusion.

[0025] Another area of related prior art to be considered is that ofdevices designed to be placed within the vertebral interspace followingthe removal of a damaged disc, and seeking to eliminate further motionat that location.

[0026] Such a device is contained in U.S. Pat. No. 4,501,269BAGBY—describing an implantable device and limited instrumentation. Themethod employed is as follows: a hole is bored transversely across thejoint and then a hollow metal basket of larger diameter than the hole isthen pounded into the hole and then filled with the bone debrisgenerated by the drilling.

[0027] While the present invention (device, instrumentation, and method)may appear to bear some superficial resemblance to the BAGBY invention,it is minimal, while the differences are many fold and highlysignificant. These differences include the following:

[0028] 1. Safety—The present invention provides for a system ofcompletely guarded instrumentation so that all contiguous vitalstructures (e.g., large blood vessels, neural structures) are absolutelyprotected. Said instrumentation also makes overpenetration by the drillimpossible. Such overpenetration in the cervical spine, for example,would result in the total paralysis or death of the patient. In thethoracic spine, the result would be complete paraplegia. In the lumbarspine, the result would be paraplegia or a life-threatening perforationof the aorta, vena cava, or iliac vessels. The present invention isatraumatically screwed into place while the BAGBY device, incontradistinction, is pounded into position. BAGBY describes that theimplant is significantly larger in size than the hole drilled and mustbe pounded in. This is extremely dangerous and the pounding occursdirectly over the spinal cord which is precariously vulnerable topercussive injury. Furthermore, while it is possible, for example in thelumbar spine, to insert the present invention away from the spinal cordand nerves, the BAGBY device must always be pounded directly towards thespinal cord.

[0029] Furthermore, since the BAGBY device is pounded into a smooth holeunder great resistance, and lacking any specific design features tosecure it, the device is highly susceptible to forceful ejection whichwould result in great danger to the patient and a clinical failure. Thepresent invention, in contradistinction, is securely screwed into place,and possesses highly specialized locking threads to make accidentaldislodgement impossible. Because of the proximity of the spinal cord,spinal nerves, and blood vessels, any implant dislodgement as mightoccur with the BAGBY device might have catastrophic consequences.

[0030] 2. Broad applicability—The BAGBY device can only be inserted fromthe front of the vertebral column, however, the present invention can beutilized in the cervical, thoracic, and lumbar spine, and can beinserted from behind (posteriorly) in the lumbar spine. This is of greatimportance in that the purpose of these devices is in the treatment ofdisc disease and probably greater than 99 percent of all lumbaroperations for the treatment of disc disease are performed from behindwhere the present invention can easily be utilized, but the BAGBYdevice, as per his description, cannot.

[0031] 3. Disc removal—The BAGBY invention requires the complete removalof the disc prior to the drilling step, whereas the present inventioneliminates the laborious separate process of disc removal andefficiently removes the disc and prepares the vertebral end plates in asingle step.

[0032] 4. Time required—The present invention saves time over the BAGBYinvention in that time is not wasted laboring to remove the disc priorto initiating the fusion. Also, since with the present invention theprocedure is performed through a system of guarded instrumentation, timeis not wasted constantly placing and replacing various soft tissueretractors throughout the procedure.

[0033] 5. Implant stability—Dislodgement of the implant would be a majorsource of device failure (an unsuccessful clinical result), and mightresult in patient paralysis or even death. As discussed, the BAGBYdevice lacks any specific means of achieving stability and since it ispounded in against resistance to achieve vertebral distraction, it issusceptible to forceful dislodgement by the tendency of the twodistracted vertebrae, to return to their original positions squeezingout the device. The present invention however is screwed into place. Asthere is no unscrewing force present between the vertebrae andcompression alone cannot dislodge the implant, the implant is inherentlystable by its design. Furthermore, the threads of the present inventionare highly specialized in that they are periodically interrupted suchthat the tail ends of each of the tabs so formed are blunted and twistedso as to resist accidental unscrewing. The removal of an implant withsuch “locking threads” requires the use of a special extractor includedwithin the instrumentation. The stability of the present invention isstill further enhanced, again in contradistinction to the BAGBY device,by the presence of a “bone ingrowth” surface texturing, which bothincreases the friction of the fit and allows for the direct growth ofthe vertebral bone into the casing of the implant itself.

[0034] 6. Spinal stability—The present invention is not onlyself-stabilizing, it also provides stability to the adjacent vertebraein at least three ways that the BAGBY device cannot. First, the BAGBYdevice is placed transversely across the joint in the center, leavingboth vertebrae free to rock back and forth over this round barrel shapedaxis, much like a board over a barrel, being used for a seesaw.

[0035] Secondly, as the BAGBY device lacks any specific design featuresto resist sliding, it may actually behave as a third body allowing thetranslation of the vertebrae relative to the device and to each other.

[0036] Thirdly, any device can only provide stability if it remainsproperly seated. The present invention is inherently stable, andtherefore assures that it will stabilize the adjacent vertebrae; ratherthan, as with the BAGBY device, where the instability of the spine to betreated may instead cause a dislocation of the implant, with furtherloss of spinal stability.

[0037] 7. The collapse of the interspace—While both the presentinvention and the BAGBY device can be fabricated to withstand thecompression forces within the interspace, the interspace maynevertheless collapse under the superincumbent body weight as theimplant settles into the vertebral bone. This is related to the load perunit area. Again the present invention is superior to the BAGBY devicein at least four ways. First, the present invention offers considerablygreater surface area to distribute the load. Secondly, while the BAGBYdevice is placed centrally, the present device is placed bilaterallywhere the bone tends to be more cortical and much stronger out towardsthe rim. Thirdly, the present invention supports the load achieving an“I” beam effect, whereas the BAGBY implant does not. Fourthly, it is notpressure alone that causes the collapse of the bone adjacent to theimplant, but also bony erosion that is caused by the motion underpressure of the implant against the bone. As discussed in item #6 above,the present invention alone is highly resistant to such motion, againdiminishing the likelihood of erosion and interspace collapse.

[0038] 8. Bone ingrowth surface texturing—The present invention has asurface treatment of known and conventional technology to induce thegrowth of bone from the vertebrae directly into the casing material ofthe implant itself. The BAGBY device has no similar feature.

[0039] 9. Fusion mass—The BAGBY invention calls for removing the discand then drilling a hole between the adjacent vertebrae. The bony debrisso generated is then put into the device. The present invention takes acore of pure bone producing marrow from the iliac crest, and then by useof a special press forcibly injects the device with an extremely densecompressed core of that osteogenic material until the material itselfvirtually extrudes from every cell of the implant.

[0040] 10. The probability of achieving fusion—The fusion rate withinthe spine is known to be related directly to the amount of exposedvascular bone bed area, the quality and quantity of the fusion massavailable, and the extent of the stabilization obtained with all otherfactors being held constant. It would then be anticipated, that thefusion rate would be superior with the present invention as compared tothe BAGBY device, because of optimal implant stability (#5), optimalspinal stability (#6), bone ingrowth surface treatment (#8), superiorfusion mass (#9), and the greater exposed vertebral bony surface area(#7).

[0041] The last area of prior art possibly related to the presentinvention and therefore, to be considered related to “BONY INGROWTH,”and patents either describe methods of producing materials and/ormaterials or devices to achieve the same. Such patents would include:

[0042] U.S. Pat. No. 4,636,526 (DORMAN), U.S. Pat. No. 4,634,720(DORMAN), U.S. Pat. No. 4,542,539 (ROWE), U.S. Pat. No. 4,405,319(COSENTINO), U.S. Pat. No. 4,439,152 (SMALL), U.S. Pat. No. 4,168,326(BROEMER), U.S. Pat. No. 4,535,485 (ASHMAN), U.S. Pat. No. 3,987,499(SCHARBACH), U.S. Pat. No. 3,605,123 (HAHN), U.S. Pat. No. 4,655,777(DUNN), U.S. Pat. No. 4,645,503 (LIN), U.S. Pat. No. 4,547,390 (ASHMAN),U.S. Pat. No. 4,608,052 (VAN KAMPEN), U.S. Pat. No. 4,698,375 (DORMAN),U.S. Pat. No. 4,661,536 (DORMAN), U.S. Pat. No. 3,952,334 (BOKROS), U.S.Pat. No. 3,905,047 (LONG), U.S. Pat. No. 4,693,721 (DUCHEYNE), and U.S.Pat. No. 4,070,514 (ENTHERLY).

[0043] However, while the present invention would utilize bone ingrowthtechnology, it would do so with conventional technology.

[0044] In summary then, the present invention, instrumentation, andmethod, alone provides for a one stage discectomy, fusion and interbodyinternal spinal fixation; that being performed more quickly, withgreater safety, and more effectively, for all of the aforementionedreasons, than is possible with any other known art.

BRIEF SUMMARY OF THE INVENTION

[0045] The present invention comprises a series of artificial implants,the purpose of which is to participate in, and directly cause bonefusion across an intervertebral space following the excision of adamaged disc. Such implants are structurally load bearing devices,stronger than bone, capable of withstanding the substantial forcesgenerated within the spinal interspace. Such devices have a plurality ofmacro sized cells and openings, which can be loaded with fusionpromoting materials, such as autogenous bone, for the purpose ofmaterially influencing the adjacent vertebrae to form a bony bond to theimplants and to each other. The implant casing may be surface texturedor otherwise treated by any of a number of known technologies to achievea “bone ingrowth surface” to further enhance the stability of theimplant and to expedite the fusion.

[0046] Further, said devices are so configured and designed so as topromote their own stability within the vertebra interspace and to resistbeing dislodged, and furthermore, to stabilize the adjacent spinalsegments.

[0047] The apparatus for preparing the vertebrae for insertion of theimplant is also disclosed, such instrumentation and method allowing forthe rapid and safe removal of the disc, preparation of the vertebrae,performance of the fusion, and internal stabilization of the spinalsegment.

Discussion of the Instrumentation

[0048] The concept of performing various aspects of this surgery are notentirely new. Drills are frequently placed through hollow, tubularguards to protect the adjacent soft tissues. A set of instrumentsdeveloped by Ralph Cloward utilizes such a tubular drill guard on alarger scale, for the purpose of drilling into the cervical spine.However, this inventor is unaware of any set of instruments, system, orprocedure designed to allow the entire surgical procedure beyond theinitial exposure, to be performed blindly and with complete safetythrough a fixed sheath apparatus. Specific design features which combineto make this uniquely possible are as follows:

[0049] 1. The availability of the specific implant.

[0050] 2. The end of all the penetrating instrumentation is blunt faced.

[0051] 3. All of the instruments have been stopped out at apredetermined depth to avoid overpenetration.

[0052] 4. The design of the external sheath conforms to the spaciallimitations of the specific surgical site.

[0053] 5. The design and use of a second or inner sheath allows for thedifference in size between the inside diameter of the outer sheath, andthe outside diameter of the drill itself. This difference beingnecessary to accommodate the sum of the distraction to be produced, andthe depth of the circumferential threading present on the implant.

[0054] 6. A specially designed drill bit with a central shaft recessallows for the safe collection of the drilling products, which can thenbe removed without disturbing the outer sheath by removing the drill bitand inner sheath as a single unit.

[0055] 7. A specially designed trephine for removing a core of boneslightly smaller in diameter than the internal diameter of the implantcavity itself, however of a greater length.

[0056] 8. A specially designed press for forcefully compressing andinjecting the long core of autogenous bone into the implant, such thatit extrudes through the implant itself.

[0057] 9. A specially designed driver extractor, which attaches to theimplant and allows the implant to be either inserted or removed withoutitself dissociating from the implant, except by the deliberatedisengagement of the operator.

OBJECTS OF THE PRESENT INVENTION

[0058] It is an object of the present invention to provide an improvedmethod of performing a discectomy, a fusion, and an internalstabilization of the spine, and specifically, all three of the abovesimultaneously and as a single procedure.

[0059] It is another object of the present invention to provide animproved method of performing a discectomy, a fusion, and an internalstabilization of the spine, which is both quicker and safer than ispossible by previous methods.

[0060] It is another object of the present invention to provide animproved method of performing a discectomy, a fusion, and an internalstabilization of the spine, to provide for improved surgical spinalimplants.

[0061] It is another object of the present invention to provide animproved method of performing a discectomy, a fusion, and an internalstabilization of the spine, which provides for an improved system ofsurgical instrumentation to facilitate the performance of the combineddiscectomy, fusion, and internal spinal stabilization.

[0062] These and other objects of the present invention will be apparentfrom review of the following specifications and the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0063]FIG. 1 is a perspective view of the driver and sheath and itsorientation to a vertebral structure.

[0064]FIG. 1A is a perspective view of the driver member for the outersheath.

[0065]FIG. 2 is a perspective view of the outer sheath being insertedinto the vertebrae structure.

[0066]FIG. 3 is a perspective view of the outer sheath and inner sheathassembly, with the drill bit of the present invention.

[0067]FIG. 3A is a side sectional view of the collar and drill bit ofFIG. 3.

[0068]FIG. 4 is a perspective view of a cylindrical implant andvertebrae structure.

[0069]FIG. 4A is a perspective view of one preferred embodiment of theimplant.

[0070]FIG. 4B is a cross sectional view of the implant of FIG. 4A.

[0071]FIG. 4C is the driving and insertion equipment for the implant ofFIG. 4A.

[0072]FIG. 4D is a side sectional view of the driver and implant betweenvertebrae.

[0073]FIG. 5 is a sectional view of the vertebrae structure, taken alonglines 5-5 of FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

[0074] Referring to FIG. 1 a vertebrae structure comprising twovertebrae V and a disc D between the two vertebrae, is shown. A hollowtubular drill sleeve 10 has teeth 12 at its lower end. The sleeve 10 hasan enlarged diameter upper portion 14.

[0075] A driver 16, shown in FIG. 1A, consists of a solid tubular member18 and an increased diameter head 20. The external diameter of the solidtubular member 18 is slightly smaller than the inside diameter of thehollow tubular drill sleeve 10 and has a length that is substantiallyshorter than the overall length of the hollow tubular drill sleeve 10.

[0076] The drill sleeve 10 is made of metal in order to be driven intothe vertebrae V and be held in place by the teeth 12 of the drill sleeve10.

[0077] Referring to FIG. 2 the drill sleeve 10 with the driver 16installed is shown being driven into two vertebrae V on either side of adisc D by hammer H.

[0078] Referring to FIGS. 3 and 3A the drill assembly is shown. In FIG.3 the drill sleeve 10 is illustrated in the two vertebrae V, straddlingthe disc D.

[0079] The retaining sleeve 15 has an outside diameter slightly smallerthan the inside diameter of the drill sleeve 10, and a lengthsubstantially the same length as the drill sleeve 10. The retainingsleeve 15 has a collar 17 at its upper end for engaging the top of thedrill sleeve 10.

[0080] The drill 22 comprises an upper portion 24, a central recessedportion 26 and a lower cutting drill portion 28. The upper 24 and lowerportion 28 of the drill 22 have the same outside diameter. The drill 24has a collar 30 attached to the upper portion 24 of the drill 22.

[0081] The outside diameter of the drill 22 is slightly smaller than theinside diameter of the retaining sleeve 15. The length of the drill,from the collar 30 to the end of the drill bit, is such that apredetermined portion of the drill bit 22 extends beyond the end 29 ofthe sleeve when fully inserted.

[0082] Referring to FIG. 4, a cylindrical embodiment of the presentinvention is shown, one implant positioned in the opening in thevertebrae and disc formed by the drill 22, and a second implant shownprior to implantation.

[0083] The cylindrical implant 50 comprises a hollow tubular memberwhich in the preferred embodiment is made of an ASTM surgicallyimplantable material, and preferably Titanium. The cylindrical implant50 is closed at one end 52 and open at the other end 54. The outercylindrical implant 50 has a series of macro-sized openings 56 throughthe side walls of the cylindrical member 50. A series of externalthreads 53 are formed on the circumference of the cylindrical implant50. The threads 53 are locking threads having a series of interjections,the ends of which are blunted and twisted so as to resist unscrewing.

[0084] The open end 54 of the cylindrical implant 50 has an internalthread 51 for receiving a complementary cap 52 which has an externalthread 58 for engaging the internal threads 51 of the cylindrical member50. The cap 52 has a hexagonal opening 59 for use with an Allen wrenchfor tightening the cap. A driver engaging element 70 is located on therear surface 60 of the implant. The driving engaging element 70comprises a raised rectangular portion 63 and a central threaded opening65, for engaging the driver apparatus, shown in FIG. 4C and FIG. 4D. Thedriving equipment 100 comprises a central tubular rod 102 having athread fitting into opening 65 in the implant. An enlarged knurled knob106 is affixed to the other end of the rod 102 for ease in turning. Thecentral rod 102 is enclosed within a hollow tubular member 108, having anarrow lower portion 110 and an increased diameter upper portion 112. Atthe end of the lower portion 110 is an attachment member 114, having agenerally rectangular depression 116 for complementing the driverengaging element 70 of the implant 50. A pair of handles 118 and 120extend perpendicular from the upper position 112 of the tubular member108 to assist in turning the driver 100.

[0085] The operation is performed in the following manner: (ExampleLumbar Spine Posterior Approach) A skin incision is made directly overthe interspace to be operated on. The dissection is carried down alongside of the superspinous and intraspinous ligaments preserving thosestructures. A semi hemi laminotomy is performed at the upper level,removing sufficient bone to allow access into the interspace. Theligament flavum is removed and then the dural sac is protected byretracting it medially along with the traversing (inferior) nerve root.The superior nerve root or the root exiting beneath the pedicle at thelevel above is visualized and protected.

[0086] At this time the drill sleeve 10 is placed into the spinal canalwith both nerve roots directly inspected and protected. The drill sleeve10 is embedded by teeth 12 spanning the disc space from the midline overand it is seated into the two vertebrae V across the disc D space byusing a driver 20. Once this is done, the driver 20 is removed and aretraining sleeve 15 is placed through the drill sleeve 10. Once seated,sleeve 10 provides absolute protection to the dural sac and nerve rootsas the remaining surgery is performed entirely through this sleeve.

[0087] The inner sleeve allows for the difference between the outsidediameter of the drill 22 and the outside diameter of the threads 53 ofthe cylindrical implant 50. This then makes it possible to perform theentire operation through the lumen of the embedded outer sleeve despitethe differences in diameter between the drill and the implant.

[0088] A drill 22 is then placed in the retaining sleeve 17. The drill22 is of such a length that it cannot penetrate more than 28 millimetersbeyond the end of the drill sleeve 10. This, of course, could be variedand made smaller for enhanced safety. However, at the present time 27 to28 millimeters seems to be safe for probably 3 standard deviations ofthe population. The drill 22 is attached to a power unit and thedrilling takes place.

[0089] The recessed central area between the reduced portion 26 allowsfor the accumulation of the debris generated by the drilling. At thistime, leaving the outer sleeve firmly embedded, the retaining sleeve 17is removed with the drill 22 as a single unit. All the vertebrae anddisc debris that was generated during the drilling is contained withinthe recess and against the inside wall of the retaining sleeve 17 andcannot come out within the spinal canal. Once the retaining sleeve 17and drill 22 is out of the patient's operative field, all of thematerial so generated can be removed.

[0090] The next step is that a screw tap is put down through the drillsleeve 10. The tap also has a collar on it that will automatically stopthe tap from extending beyond 28 millimeters of penetration. The tapitself has a blunt nose that would also avoid any perforation. The tapis then removed. The tap size has deliberately been selected so that itsinner root diameter is 1.3 millimeters greater than the outside diameterof the drill 22. This insures that the interspace will be distracted byat least that much once the implant is placed. The tap has its outsidediameter 1.2 millimeters greater than its root diameter. The tap isremoved and the space is now prepared to accept the cylindrical implant50.

[0091] The implant 50 is prepared by utilizing the trephine, a hollowdrill, to obtain a core of pure cancellous bone from the patients iliaccrest of slightly smaller diameter than the internal diameter of theimplant but approximately 6 mm longer. The implant 50 is placed in apress like device like an ammo loader and the bone graft measuringapproximately 32 millimeters is then compressed into the hollow body ofthe implant (26 mm) so that the bone graft fills the opening 54 andextends through the openings 56. The cap 60 is then screwed on to theimplant 50 by use of an Allen driver/wrench, and the device is ready forimplantation.

[0092] The inserter/remover is such that it locks onto the implant, sothat the implant can be moved either clockwise or counter-clockwise,screwed or unscrewed. The implant itself has for its root diameter thesame exact root diameter as the tap which as already noted is already1.3 millimeters greater than the drill and has an outside diameter, 1.5millimeters greater than its root. This is also 0.3 millimeters greaterthan the threads cut by the tap so that in inserting the device it isactually cutting through previously uncut bone, helping to insure thatit locks in firmly. The threads on the implant 50 are locking threads sothat it is easier to screw the device in than for it to be unscrewed.However, with sufficient torque it is possible to extract the device ifone so desires.

[0093] Once the implant has been seated it is able to be inserted only28 millimeters. Since the implant 50 is only 26 millimeters in length,this virtually guarantees that the implant 50 will be recessed into thevertebral bodies more than 2 millimeters and cannot protrude into thespinal canal.

[0094] Similarly, the implants shown in FIG. 4B can be implanted. Theimplant in FIG. 4B is a modified solid, having extensive channelingthroughout, and has no cap. A central opening 61 permits insertion ofthe bone graft material into the interior of the implant.

[0095] These implants have a surface configuration such as to inducebone ingrowth through the implant, and into the wall of the vertebrae ineffect inducing fusion from one vertebrae in joint to the other, therebyeventually making the implant itself superfluous as the bone would dothe work.

[0096] The implant itself, because of its being made of strongermaterial than bone, would provide structural support to the twovertebrae while awaiting bone ingrowth. Once the bone ingrowth occurred,however, the implant would be firmly and permanently fixed in place.

[0097] As shown in FIG. 4, more than one implant is inserted into thedisc space, thereby preventing the rocking motion that would result inthe difficulties referred to above in the discussion of the Bagbypatent.

[0098] While the invention has been described with regards to thepreferred embodiment, it is recognized that alternative embodiment maybe devised which would not depart from the present invention.

What is claimed is:
 1. A device for insertion between two boneystructures for maintaining spacing and facilitating bone fusioncomprising: an elongated body having an outer surface extending along alongitudinal axis between a distal end and a proximal end; a helicalthread pattern formed around at least a portion of said elongated bodyand defining a forward insertion rotation direction and a backwardunscrewing rotation direction; and means for impaling adjacent bonestructures to resist backward rotation of said elongated body once it isinserted into bone.